This invention relates generally to epitaxial growth techniques and more particularly to epitaxial growth of metals.
As is known in the art, metallic films such as iron can be grown epitaxially to form single crystal layers on substrates of gallium arsenide in the {110} orientation. The (110) face contains the magnetic easy axis &lt;001&gt;, the magnetic axis hard &lt;111&gt;, and the intermediate axis &lt;11022 . In an article entitled "Molecular Beam Epitaxial Growth of Single Crystal Fe Films on GaAs", by Prinz et al, Applied Physics Letter 39 (V) September 1981, the authors describe using molecular beam epitaxy (MBE) to grow single crystal iron films over (110) faces of gallium arsenide. In the molecular beam epitaxy technique as described in the above article, a source of iron is heated to a high temperature (1150.degree. C.) and iron atoms are vaporized over the iron source. In the presence of an ultra-high vacuum, characteristic of the molecular beam epitaxy approach, the vaporized iron atoms migrate towards the substrate. The substrate is generally heated to a temperature in the range of about 50.degree. C. to 450.degree. C. With this technique, the amount of kinetic energy imparted to the iron atoms is related to the amount of kinetic energy imparted to the iron atom during heating of the iron source, and the temperature of the substrate. Typically, the kinetic energy of these iron atoms is in the range of 0.1-0.2 eV.
While growth in the {110} family of planes provide interesting material for use in metallic studies, it is believed the growth in the {100} family of planes, for example the (100) face generally used for device fabrication in gallium arsenide, would be more beneficial. The {100} family of planes include the (100), (010) and (001) faces of the cubic zinc blende gallium arsenide crystal. For example, it has been suggested that multi-layer semiconductor structures may be ultimately fabricated. Such multi-layer structures would involve forming multi-semiconductor epitaxial layers each having devices fabricated thereon, with said devices being interconnected by metallic conductors. Over underlying epitaxial semiconductor layers would then be grown subsequent epitaxial semiconductor layers. In order for the subsequent semiconductor layers to have the same crystal structure as the underlying epitaxial layers, the metallic interconnects would also have to be single crystal layers having a similar crystal orientation as that of the substrate. Therefore, since for device work, the &lt;100&gt; orientation is preferred in materials such as gallium arsenide, for example, it would be advantageous to grow metal layers over gallium arsenide likewise having the &lt;100&gt; orientation.
Furthermore, to exploit the magnetic properties of certain metals such as iron, it would also be advantageous to grow these metals such as iron in the &lt;100&gt; orientation over &lt;100 &gt; orientated semiconductor materials such as gallium arsenide. Iron grown on the (100) planes will provide mutually orthogonal, planar crystal orientations in the &lt;010&gt; and &lt;001&gt; orientation. These mutual orthogonal, planes crystal orientations or directions will provide an iron film having a natural bi-axial anisotropy and accordingly, will provide a film having a pair of magnetization states which are easy to magnetize in directions parallel to either one of the &lt;010&gt; or &lt;001&gt; directions but would be difficult to magnetize in the intermediate axies orientations such as the &lt;110&gt; orientation. This characteristic may be used to provide magnetic storage elements, as described in U.S. patent application Ser. No. 57,089 by E. Schloemann, filed on the same day as this application, and assigned to the same assignee as the present invention.
Low substrate temperatures are also desirable because a metal layer may have a substantially different thermal expansion characteristic than the substrate. Upon cooling of the substrate, these differences can lead to crystal strain causing slight misalignment of the atoms in the iron crystal layer which degrades crystal quality.
Low substrate temperatures are also required to prevent low decomposition temperature materials such as GaAs from decomposing during growth of the layer.